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How much cfm do I really need?

Old 03-23-2016, 02:38 AM
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Default How much cfm do I really need?

In the middle of rebuilding I'm stuck with some basics.

The heads of the 1994 blazers seem to suck at some 135 cfm.

When plugging in a couple of figures to try to get a hold of what cfm numbers are really required for a specific power output I'm getting to flows in the 75 - 85 cfm (theoretical) for a 250 - 275 hp 4.3l V6 (262 cid).
That would mean:
- the OEM heads would be perfectly fine to make that hp.
- at least half of the tuning for cfm on the heads is overkill.

I'm aware of the following:
- a high average airflow at half lift makes most power and especially torque
- the time is the restricting factor on a valvetrain but I'm aiming at around 3'000 - 3'250 rpm for peak torque
- even at 135 cfm below the opening curve of the valvetrain I should be able to get to an average of 75 cfm.

Is there somebody which can point me to the right sources of wisdom as to required cfm per cylinder?
Anybody ever tuned a 4.3l and what were the figures?

I guess that for serious hp I would need a head like BRODIX -6's or swap a 1996 or later kidney bean combustion chamber head on.
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